Generally, a refrigerant circuit of a refrigeration cycle apparatuses has a structure in which a compressor for compressing a refrigerant, a gas cooler for cooling the refrigerant, an expansion valve for expanding the refrigerant and an evaporator for heating the refrigerant are connected in this order. In the refrigeration cycle of such a refrigerant circuit, the refrigerant undergoes a pressure drop from high pressure to low pressure at the expansion valve while being expanded, and an internal energy is released at that time. The internal energy to be released increases as a pressure difference between a low pressure side (evaporator side) and a high pressure side (gas cooler side) of the refrigerant circuit increases, lowering the energy efficiency of the refrigeration cycle.
In view of such a problem, a variety of techniques have been proposed for recovering the internal energy of the refrigerant released at an expander. JP 2004-44569 A, for example, proposes a technique for recovering energy by coupling a rotating shaft of a rotary type expander to a rotating shaft of a motor for driving a compressor.
FIG. 26 is a configuration diagram of a conventional refrigeration cycle apparatus 501 that recovers energy by coupling a shaft 507 of an expander 504 to a rotating shaft of a motor 506 for driving a compressor 502.
As shown in FIG. 26, the refrigeration cycle apparatus 501 includes a refrigerant circuit in which a gas cooler 503, the expander 504, an evaporator 505, and the compressor 502 are connected in this order. The expander 504 is a rotary type or scroll type expander having a shaft 507 as a rotating shaft. The shaft 507 is coupled to the motor 506 driving the compressor 502. Rotation energy (mechanical power) of the shaft 507 is transferred to the rotating shaft of the motor 506. Thus, a part of the internal energy released when the refrigerant undergoes a pressure drop from high pressure to low pressure at the expander 504 while being expanded is converted into the rotation energy of the shaft 507, transferred to the motor 506, and then is utilized as a part of mechanical power for driving the compressor 502. Accordingly, the refrigeration cycle apparatus 501 can realize high energy efficiency.
JP 57 (1982)-108555 A discloses a technique for recovering energy from a refrigerant using a medium-driven motor having no specific volumetric capacity ratio (an expansion ratio). FIG. 30 is a diagram showing the structure and operation principle of the medium-driven motor disclosed in JP 57 (1982)-108555A. A medium-driven motor 700 includes a cylinder 701, a rotor 702 (a piston) that rotates in the cylinder 701, and a vane 705 that divides a working chamber formed between the cylinder 701 and the rotor 702 into a suction side working chamber 706a and a discharge side working chamber 706b. The cylinder 701 has a suction port 703 so that a refrigerant can be drawn into the suction side working chamber 706a, and a discharge port 704 so that the refrigerant can be discharged from the discharge side working chamber 706b. Neither the suction port 703 nor the discharge port 704 has a valve, but the shape of the rotor 702 is determined to prevent the refrigerant from flowing from the suction port 703 to the discharge port 704 directly. Specifically, a part of an outer peripheral face of the rotor 702 has the same curvature radius as that of an inner peripheral face of the cylinder 701.
JP 2006-266171 A also discloses a technique for recovering mechanical power from a refrigerant. JP 2006-266171 A proposes a technique for recovering mechanical power by coupling a rotating shaft of a sub compressor provided on a suction side of a compressor to a rotating shaft of a rotary type expander.
FIG. 27 is a configuration diagram of a power-recovery-type refrigeration cycle apparatus 601 using an expander-compressor unit 608, described in JP 2006-266171 A. As shown in FIG. 27, the refrigeration cycle apparatus 601 includes a refrigerant circuit in which a sub compressor 602, a main compressor 603, a gas cooler 604, an expander 605, and an evaporator 606 are connected in this order.
FIG. 28 is a cross-sectional view of the expander-compressor unit 608. As shown in FIG. 28 and FIG. 27, the expander-compressor unit 608 is composed of the sub compressor 602 and the expander 605 sharing a rotating shaft 607. Thus, energy recovered by the expander 605 is supplied to the sub compressor 602 via the rotating shaft 607, and is utilized as a driving force for the sub compressor 602. Accordingly, the refrigeration cycle apparatus 601 shown in FIG. 27 can realize high energy efficiency.
FIG. 29 is a cross-sectional view of the expander 605. As shown in FIG. 29, the expander 605 is a swing type expander in which a piston 611a and a vane 611b are formed integrally. A shoe 612 is attached to the vane 611b. The shoe 612 has a narrow refrigerant passage 613 that communicates with a working chamber 614. In the expander 605, the vane 611b reciprocates, and the shoe 612 swings. The refrigerant passage 613 is opened and closed corresponding to the reciprocating motion of the vane 611b and the swinging motion of the shoe 612, and thereby timing for drawing the refrigerant is controlled.
The expanders disclosed in JP 2004-44569 A and JP 2006-266171 A each have a specific volumetric capacity ratio (a ratio of a discharge volume to a suction volume). Thus, in the expanders disclosed in JP 2004-44569 A and JP 2006-26617 A, a discharge pressure is determined automatically from a suction pressure and the volumetric capacity ratio of each of the expanders. However, the high pressure and low pressure of the refrigeration cycle vary, respectively, depending on its operating conditions. Accordingly, the discharge pressure of the expander (the pressure of the refrigerant being discharged from the expander) does not agree with the low pressure of the refrigeration cycle in some cases. For example, there arises a problem that overexpansion loss occurs when the discharge pressure of the expander becomes lower than the low pressure of the refrigeration cycle, lowering the efficiency in recovering the internal energy of the refrigerant at the expander.
That is, use of the expanders disclosed in the aforementioned documents makes it difficult to recover efficiently the internal energy of the refrigerant.
Moreover, the expander 605 shown in FIG. 28 and FIG. 29 has a complicated configuration, and is disadvantageous in terms of cost and productivity. In the expander 605, the narrow refrigerant passage 613 needs to be formed in the shoe 612 that swings. Thus, use of the expander 605 complicates the configuration of the refrigeration cycle apparatus, and tends to cause increased cost and reduced productivity.
Since the medium-driven motor 700 shown in FIG. 30 has no specific volumetric capacity ratio (the volumetric capacity ratio thereof is 1), the efficiency in recovering energy from the refrigerant hardly is affected by the pressure condition of the refrigeration cycle. Moreover, the cost and productivity problems hardly arise because it has a simple structure. In the medium-driven motor 700, however, a state in which a single working chamber 706 is formed in the cylinder 701 lasts for approximately 90° in terms of rotation angle of the rotor 702, as shown in Step 4 and Step 5 of FIG. 30. Moreover, as known from Step 5, a period during which both of the suction port 703 and the discharge port 704 are closed by the rotor 702 is relatively long. Thus, when the medium-driven motor 700 is included in the refrigerant circuit as a power recovery means, pulsation of the refrigerant in the refrigerant circuit becomes extremely strong, causing noise and vibration. Lubrication failure also tends to occur on the piston.